Method of propulsion employing catalyzed nitroalkane-alkylene oxide composition



q 3 98,350 Patented July 23, 1963 3,093,350 METHOD OF PRSPULSIGN EMPLUYING QATALYZED NITROALKANE ALKYLENE QXIDE COMPOSlTiON John 1?. Barry, Glendora, and Herman Schneiderman,

West Covina, Calif., assignors to Aerojet-General Corporation, Azusa, Califi, a corporation of Ohio No Drawing. Filed Feb. 14, 1955, Ser. No. 488,155 15 Claims. (Cl. 6035.4)

This invention relates to fuels useful in jet, ram-jet and rocket engines, as well as liquid guns. In particular, it relates to fluid monopropellant fuels which are stable toward shock and high rates of compression, have low freezing points and exhibit improved burning characteristics.

A monopropellant fuel refers to substances containing sufficient oxygen in their structure to sustain combustion without relying on an external supply of oxygen. Such monopropellants find valuable use in capacities where high thrust per unit weight is important, as for example, in propelling jet aircraft, rockets, underwater missiles, and the like. The monopropellants of this invention are particularly useful in motors of the type disclosed in assignees copending application Serial No. 183,111, filed September 5, 1950.

At this present time, liquid nitroalkaues are widely used as monopropellants. Unfortunately, these fuels are extremely sensitive to shock, and detonate at compression rates as low as 20,000 p.s.i./sec. It is also characteristic of these nitroalkaues that once detonation occurs, it readily propagates through the fuel. Hence, detonation of a nitroalkane in an engine is extremely hazardous, as the detonation is usually transmitted through the fuel lines to the main fuel storage supply. In contrast to their sensitivity toward detonation, the nitroalkaues burn very slowly. Due to this slow burning rate, motors in which nitroalkanes are used as fuels must have large combustion chamber volumes to permit eflicient utilization of the fuel. The size of these motors precludes their use where weight limitations are critical. In addition to exhibiting undesirable explosive and burning properties, these nitroalkaues have relatively high freezing points which often render them impractical for arctic or high altitude use.

Various additives have been used with nitroalkaues in an effort to improve their burning and explosive properties. For example, it is known that the addition of 1% by weight of pyridine salt of uranyl acetylacetonate raises the threshold explosion temperature of nitromethane to 615 F. and the addition of 1% by weight dicyclohexylamine chromate raises the threshold explosion temperature to 691.7" F. However, none of the known additives have materially reduced the sensitivity of the nitroalkanes toward shock or compression or reduce the tendency of the nitroalkaues to propagate detonation. -Nor do any of the known additives improve the burning characteristics of the nitroalkaues so that they may be used in small motors. In addition, none of the known additives can be used in suflicient quantity to substantially reduce the freezing point of the nitroalkaues without destroying the propellant properties of the fuel.

We have now found that a fuel comprised of alkylene oxide, liquid nitroalkane, and a combustion rate acceleration catalyst, exhibits monopropellant properties substantially equivalent to the pure nitroalkane; possesses stability toward high compression rates; and does not propagate detonation. Fuels of this type exhibit lower freezing points and viscosity than corresponding nitroalkaues and can be used at markedly lower combustion chamber pressures and in combustion chambers of lower L (characteristic combustion chamber length) than the corresponding nitroalkaues. In addition, its low sensitivity to guning them together.

fire makes this fuel especially valuable in military aircraft.

According to the present invention, the fuel is prepared by blending together alkylene oxide, nitroalkane and a combustion rate acceleration catalyst. The components of the fuel are readily compatible and the alkylene oxide and the liquid nitroalkane can be blended by merely pour- The combustion rate acceleration catalyst readily dissolves in the fuel mixture. Optimum results are obtained when the fuel contains from about 10% to about 96% by weight nitroalkane, from about 3% to about alkylene oxide and from about 1% to about 5% combustion rate acceleration catalyst.

The liquid nitroalkaues useful in the practice of this invention are generally those having from 1 to 6 carbon atoms. However, mixtures of liquid nitroalkaues and solid nitroalkaues dissolved in liquid nitroalkaues can also be used. Due to their low molecular weight, nitromethane and nitroethane are ordinarily preferred. Examples of other nitroalkaues useful as fuel components are: dinitromethane, tetranitromethane, dinitroethane, mononitropropane, dinitropropane, trinitropropane, mononitrobutane, dinitrobutane, trinitrobutane, tetranitrobutane, dinitropentane, tetranitropentane,nitrated hexane, and mixtures thereof.

The alkylene oxide component is a compound or compounds having an oxygen atom bonded to two adjacent and mutually bonded carbon atoms such as ethylene and propylene oxide, which are generally preferred due to their low molecular weight and greater economic availability.

Combustion rate acceleration catalysts used in the novel fuels of this invention are organo-coordination compounds of chromium, molybdenum, tungsten, manganese, cobalt, nickel, iron and platinum. Due to their stability, the lower diketone coordination compounds of the above metals are preferred. Among the diketone-coordination compounds useful as catalysts are: chromium acetylacetonate, molybdenum acetylacetonate, tungsten acetylacetonate, manganese acetylacetonate, cobalt acetylacetonate, nickel acetylacetonate, ferric acetylacetonate and platinum acetylacetonate.

To illustrate the features of the fuel, the properties of a typical mixture composed of 79% nitromethane, 19% ethylene oxide and 2% chromium acetylacetonate, and the corresponding properties of nitromethane, are tabulated below in Table I.

Table I 79% Nitromethane, Nitromethane 19% Ethylene oxide, 2% Chromium acetylacetonate Viscosity at 0 F., 0.89.

centistokes. Freezing point, F 45. Adiabatic compres- 80, 000.

sion rate (50% explosion probability), p.s.i./scc.

Sensitivity to ex- Initiatedby #8 electric Seldom initiated by #8 plosive shock. blasting cap. blasting cap; and then only partially. Sensitivityto guufire Detonated by gunfire Did not detonate when while contained in subjected to gunfire tanks having 46" While contained in steel walls. tanks having steel walls. Tendency to explo- Propagated in di- Did not propagate in diameter stainless steel tubing.

ameter stainless sive propagation.

steel tubing.

mium acetylacetonate. The properties of this particular fuel are presented in Table I I.

3 Table II 59% Nitromethane, 39% Ethyl- The above fuels are thermally stable at temperatures at least as high as 698 F. and can be stored for long periods of time without deterioration.

The new fuel of this invention is employed in jet and rocket engines by merely injecting it into the combustion chamber and igniting it therein by conventional means. The gases produced by the combustion escape to the atmosphere through a nozzle to create thrust. In addition to the desirable physical properties described above, the fuel is capable of delivering a high performance will op erating with a much lower combustion chamber pressure and smaller characteristic combustion chamber length than is possible when employing the corresponding nitroalkane alone. For example, nit-romethane burning in a 520" rocket thrust chamber requires a chamber pressure of 500 p.s.i. to produce a 200 lb. thrust. The fuel composed of 79% nitromethane, 19% ethylene oxide and 2% chromium acetylacetonate, however, requires a chamber pressure of only 179 p.s.i. while producing thrust of the same magnitude, thus permitting greater safety, lower tank pressurization and lower over-all system and engine weight.

It has also been found that the fuel performs well at a characteristic chamber length as low as 98.5. The characteristic chamber length is defined in the art as the length a motor of the same volume would have if it were a straight tube of the same volume and had no converging section. The relationship is as follows:

where L" is the characteristic chamber length in feet, A; is the nozzle throat area in square feet and V is the chamber volume in cubic feet. In order to use nitromethane as a jet or rocket fuel, the characteristic chamber length of the motor must be at least 192 inches. As can readily be seen, the use of the fuels of this invention permit substantial reductions in the characteristic chamber length and hence in the weight of the motor.

The fuel according to our invention has the advantage of being a self-combustible propellant which can be practically and efficiently employed in jet and rocket engines without exposing the operator or apparatus to the danger of explosion, and to the attending danger of propagation of the explosion to the main storage supply through the fuel lines. The fuels of our invention can be used in the same manner and for the same purposes as nitroalkanes. They can also be employed under operating conditions where the use of nitroalkanes is precluded because of their explosive characteristics. Our novel monopropellant fuels can be used under extreme low temperature conditions, and their burning characteristics permit substantial reductions in the weight of the engine. The improved fuels of this invention will find valuable applications wherever weight, safety, temperature or performance requirements are important.

We claim:

1. In the method of producing thrust for propulsion by combusting a fluid propellant composition consisting essentially of a mixture of liquid nitroalkane and lower alkylene oxide in a combustion chamber; the improvement which comprises combusting the fluid propellant composition in the presence of a combustion rate acceleration catalyst selected from the group consisting of the lower diketone coordination compounds of chromium, molybdenum, tungsten, manganese, cobalt, nickel, iron and platinum, and mixtures thereof.

2. The method of producing thrust for propulsion by burning a fluid propellant composition which comprises burning a propellant composition consisting essentially of a mixture of liquid nitroalkane in an amount of from about 10% to about 96% by weight and lower alkylene oxide in an amount of from about 3% to about by weight in a combustion chamber; in the presence of a combustion rate acceleration catalyst, in an amount of from about 1% to about 5% by weight of the total fuel mixture, selected from the group consisting of the lower diketone coordination compounds of chromium, molybdenum, tungsten, manganese, cobalt, nickel, iron and platinum, and mixtures thereof.

3. A fluid propellant composition consisting essentially of a mixture of liquid nitroalkane, lower alkylene oxide and a combustion rate acceleration catalyst selected from the group consisting of the lower diketone coordination compounds of chromium, molybdenum, tungsten, manganese, cobalt, nickel, iron and platinum, and mixtures thereof.

4. A fluid propellant composition consisting essentially of a mixture of from about 10% to about 96% by weight of a liquid nitroalkane selected from the group consisting of the nitrometh-anes, nitroethanes, nitropr-opanes, nitrobutanes, nitropentanes, nitrohexanes, and mixtures thereof and from about 3% to about 85 by weight of alkylene oxide selected from the group consisting of propyl ene and ethylene oxide; and from about 1% to about 5% by weight of a combustion rate acceleration catalyst selected from the group consisting of the lower diketone coordination compounds of chromium, molybdenum, tungsten, manganese, cobalt, nickel, iron and platinum, and mixtures thereof.

5. A fluid propellant composition consisting essentially of a mixture of liquid nitroalkane in an amount of from about 10% to about 96% by weight, lower alkylene oxide in an amount of from about 3% to about 85 by Weight, and a combustion rate acceleration catalyst in an amount of from about 1% to about 5% by weight, selected from the group consisting of the diket-one coordination compounds of chromium, molybdenum, tungsten, manganese, cobalt, nickel, iron and platinum, and mixtures thereof.

6. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is chromium acetylacetonate.

7. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is molybdenum acetylacetonate.

8. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is tungsten acetylacetonate.

9. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is manganese acetylacetonate.

10. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is ferric acetylacetonate.

11. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is cobalt acetylacetonate.

12. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is nickel acetylacetonate.

13. The fluid propellant composition of claim 5 wherein the combustion rate acceleration catalyst is platinum acetylacetonate.

14. A fluid propellant composition comprising a mix- References lifted in the file of this patent ture of nitromethane in an anilount of about 79% by UNITED STATES PATENTS Wei ht of the composition, ethy ene oxide in an amount of a bout 19% by weight of the composition and chro- Z Malsner 11113712 1955 mium acetylacetonate in an amount of about 2% by Zwlcky et a1 31957 weight of the composition. OTHER REFERENCES A fl Propellam composing compnsmg a Jet Propulsion Journal of the American Rocket Society, ture of mtromethane 1n an amount of about 59% by VOL 24 2 (March Apri1 1954) pp 111, 112 weight of the composition, ethylene oxide in an amount wilsmz Stability of Ethylene Oxide, JARS VOL of about 39% by Weight of the composition and chromium 10 23 (November Dccember 1953 pages acetyhcemP'flte in an amount about 2% by Weight of Robison: Thesis submitted to California Institute of the Composmon- Technology and made available to the public in June 

1. IN THE METHOD OF PRODUCING THRUST FOR PROPULSION BY COMBUSTING A FLUID PROPELLANT COMPOSITION CONSISTING ESSENTIALLY OF A MIXTURE OF LIQUID NI ROLKANE AND LOWER ALKYLENE OXIDE IN A COMBUSTION CHAMBER; THE IMPROVEMENT WHICH COMPRISES COMBUSTING THE FLUID PROPELLANT COMPOSITION IN THE PRESENCE OF A COMBUSTION RATE ACCELERATION CATALYST SELECTED FROM THE GROUP CONSISTING OF THE LOWER DIKETONE COORDINATION COMPOUNDS OF CHROMIUM, MOLYBEDENUM, TUNGSTEN, MANGANESE, COBALT, NICKEL, IRON AND PLATINUM, AND MIXTURES THEREOF. 